This invention relates generally to a power regulating system for a varying load and more particularly to an output power regulating system for a wound-rotor motor, flywheel-type load peak shaver. The purpose of the shaver is to provide locally to a cyclically varying load, such as a group of excavators, a predetermined portion of the instantaneous load power departures from average power. This relieves the utility lines of most of the short-cycle power swing magnitude, resulting in improved power plant operation, and in lower excursions of receiving-end line voltage.
Excavating machines, such as power shovels and draglines utilized by the mining industry, characteristically convert power with motor-generator sets driven by large synchronous motors, which draw and regenerate power in a cyclically varying manner during a digging cycle. For example, simultaneous acceleration of the hoist and swing motions of a large earth-moving dragline may briefly draw power as great as 165% of the rating of the synchronous motors while subsequent deceleration of the swing and hoist motions to standstill may regenerate power back into the utility lines equal to the motor rating. Thus, a peak-to-peak power swing of 265% of motor rating within a typical one-minute digging cycle is not uncommon. While such power swings can be easily absorbed by a power utility having a large generating capacity, excessive load fluctuations can cause boiler water level oscillation in steam stations when the cyclically varying load forms a significant percentage of the utility steam generating capacity. Because of the problems associated with boiler water level oscillations, e.g., carry-over of water through the turbine blades, etc., some power utilities have imposed severe penalty charges for excavator equipment tied to their power lines, where this equipment imposed short-term power swings in excess of a specified magnitude, said 15 megawatts.
The problem of cyclically varying loads produced by excavator equipment upon power lines heretofore has been attacked by various techniques in attempts to maximize utilization of utility power without incurring penalties. For example, it has been proposed to limit the maximum power flow to and from one or more excavators in a surface mine by continuously monitoring, on board each excavator, the total mine load. Excursions in mine power approaching specified limits would trigger automatic cut back in the available top speed of the excavator motion drives. This, however, places a restriction on production.
Statistical studies also have been made, where there are several excavators in one mine, to determine the probability of all excavators having synchronized severest digging cycles, such that worst-motoring peaks would occur simultaneously, as would worst-regenerative peaks. This would create the very maximum peak-to-peak mine load swing. These studies show the probability of occurrence of load swings of various magnitudes. There are so many digging cycles in one year that one chance in 10,000, say, of a severe peak-to-peak swing represents a recurring reality.
It is known to relieve the utility from large power swings created by excavator equipment through the utilization of a double-fed wound-rotor power converter to transform electrical power to kinetic energy during net regeneration from the excavator equipment. The kinetic energy then is stored by speed increase of a large flywheel mounted on the shaft of the wound-rotor converter, for transformation back to electrical energy during the subsequent motoring swing of the equipment.
A possible method for controlling the running power exchange between the load peak shaver and the power system would establish a power dead band within which the shaver would not give out or receive power. This band might be, say, plus or minus 7.5 megawatts (MW) to each side of the running average load from the mine. Mine power swings within this band would be totally absorbed by the utility. Any instantaneous power excursion in excess of 7.5 MW would ideally be totally provided by the load peak shaver. Such a system has the following deficiencies:
1. The load peak shaver is idle within the 15 MW band of power swings. Very high rate-of-change of mine power could be passed along to utility. (A better approach would be to use the shaver's capabilities partly to shave off even the smallest mine load swings.)
2. Within the dead band, there might be a large transient block of energy delivered to the mine at a power excursion level low enough so as not to exceed the dead band. The shaver speed could be creeping upwards towards it top limit during this time, since the power control issues no commands inside the dead band. A regenerative peak in excess of the dead band could then come along and call for flywheel acceleration to absorb the energy. Such an increase in speed might be in excess of the shaver rating and some suitable safeguard, such as a large dumping resistor to absorb regenerative energy in excess of that which drove the shaver speed to the top limit, might be required.